In the fabrication of printed circuit cards, via holes are formed to create passages from one face or surface of the card to the opposing face or surface of the card. In passing through the card, electrically conductive lands formed within the card during the manufacture of the card, may be penetrated or cut into thus exposing a cross-section of the land to the via hole. The land may be disposed internal to the printed circuit as on a surface of a layer, which is subsequently joined with other layers to form the card or onto the external surface of the printed circuit card.
The via holes are plated, using any of several conventional plating techniques, to deposit a thin layer of metal on the interior surface of the passage or via hole. The metal layer within the via hole is in the form of a tube formed or deposited on and making contact with the cross-section of any land penetrated by the formation of the via. The formation of printed circuit cards of the type used herein is a conventional process well known in the electronics industry and as such does not form a part of this invention.
During the plating of the via hole, a flange surrounding the end or opening of the via may be formed on at least one surface of the printed circuit card. The plating of the flange may occur simultaneously to the plating of the interior tubular plated layer within the via hole.
The flange forms a pad that serves to increase the surface area, which may be wetted with solder at a later stage, in order to connect electrical components to the metal deposited by the plating. The formation of the pad alternatively may be formed as a separate plating operation or metal deposition operation together with the formation of any land extending therefrom formed on the surface of the printed circuit board.
Plated through via holes previously have been filled to enhance solder connection to the plating. U.S. Pat. No. 4,830,264, issued to Bitaillou et al., discloses filling the hollow rivet-like structure of the via hole plating with a solder flux. The solder flux filling enhances the wetting of the plated structure by the solder when heated for reflow. Then a larger than necessary solder ball of controlled volume is deposited onto the solder flux which has been filled into the via hole; a portion of the flux resides on the surface of the electronic circuit board in the region of the via hole and on the pad surrounding it.
The solder ball is larger than that which would be necessary only to fill the via hole so that when the solder ball is reflowed the via hole will fill through capillary action and leave a solder ball on the surface of the printed circuit board sufficiently large and of the desired volume that it may be used to attach another electrical component to the circuit board.
The solder ball formed on the surface of the printed circuit card may be used to connect another electronic component to the printed circuit card by any conventional technique. The Bitaillou et al. patent illustrates the affect that is a source of the problem if the via is not adequately filled with solder prior to attachment of a solder ball connected electronic module. Should there be an attempt to connect an electronic module to the printed circuit board using solder ball connection techniques and the plated through via holes have not been filled previously with solder, then whenever the solder connection between the electronic module and the printed circuit card is reflowed, the via hole will pull molten solder into the via hole thus significantly reducing the volume of solder available for the interconnection between the electronic module and the printed circuit card. Unless the volumes of the via hole and the volume of solder in the solder ball are very accurately controlled, variations in the volume of the remaining solder ball after reflow may be the source of unreliable electrical connections between the two electronic components.
U.S. Pat. No. 4,336,551 to Fujita et al., discloses forming a via hole through a glass insulation layer and then forming a pedestal for attaching solder balls on an integrated circuit. The pedestal is formed of a silver/palladium paste. Subsequent to the formation of the silver/palladium (Ag/Pd) paste pedestal, the silver/palladium paste particles are sintered to form rigid integrity into the contact pedestal and establish electrical continuity. In the attachment of solder balls onto an integrated circuit to the sintered Ag/Pd pedestal, the pedestal serves as a contact region and does not reflow.
U.S. Pat. No. 4,383,363, issued to Hayakawa et al., uses a solder paste to interconnect the conductive planes on either face of a printed circuit board through a via hole. The via hole is not plated and the solder paste is not reflowed. Conductive layers deposited on the substrate and over the via entrap the solder paste within the via hole. Electrical continuity is established through particle-to-particle contact.
U.S. Pat. No. 5,060,844, issued to Behun et al., discloses a solder ball connect technique where the internal metallurgy of the substrate or device being connected to the substrate, is solid conductor lands and is not via holes. The Behun et al. patent addresses the solder ball connection of electronic components, wherein the connections on each of the components are to a solid electronic land.
With solid lands, the Behun et al. structure does not present the problem presented by plated through hole vias. The Behun et al. patent with its inherently solid electrically conductive lands does not wick or pull the molten solder from the attachment point into a cavity of the via hole within the printed circuit card.
IBM Technical Disclosure Bulletin, Vol. 20, No. 10, March 1978, page 3872, by E. Stephens, discloses a conductive epoxy which is used to fill via holes and which establishes electrical continuity between the external surfaces of the metallized ceramic body through which the via holes extend. IBM Technical Disclosure Bulletin, Vol. 33, No. 12, May 1991, pages 144-145, discloses a machine for positioning and melting solder wire to fill via holes on a one-at-a-time basis. This process while providing a solution to the problem of prefilling via holes with solder is not a process which lends itself to high volume throughput necessary for manufacturing.